comparing main effects in factorial model using NLMixed contrasts

I was wondering what the proper syntax is for comparing main effects in a fully factorial model in NLMixed? The model (with generic variables substituted for the real ones) is below. In this case, x1a and x1b are two levels of a 3-level categorical variable (study site), and same goes for x2a and x2b (temporal seasons). The reference levels are x1c and x2c.